Exhaust gas treatment

ABSTRACT

A device ( 1 ) is disclosed for generating gaseous ammonia formed by the hydrolysis of an aqueous solution of urea at elevated temperature and pressure and for feeding the gaseous ammonia into the exhaust gas of an IC engine as it flows through the exhaust system of the engine. The device ( 1 ) is placed in the exhaust system and has an inlet ( 2 ) for the exhaust gas and an outlet ( 3 ) for the exhaust gas so that the exhaust gas will flow through the device ( 1 ) during use. The aqueous solution of urea is contained in a reaction vessel located between the inlet ( 2 ) and the outlet ( 3 ) such that, in use, the vessel and therefore the urea solution are heated by heat exchange with the exhaust gas as it flows from the inlet ( 2 ) to the outlet ( 3 ). A pump ( 32 ) is provided for pumping urea solution into the reaction vessel and a controller ( 33 ) controls the pump ( 32 ) in response to changing NOx output from the IC engine such that, in response to an increase in NOx output, the controller ( 33 ) controls the pump ( 32 ) to increase the level of urea solution in the reaction vessel, thereby increasing the surface area of urea solution available for heat exchange with the exhaust gas so as to increase the rate of production of gaseous ammonia in the reactor vessel.

The present invention relates to the control of an apparatus for reducing emissions of Nitrogen oxides (NOx) in exhaust gasses of an internal combustion (IC) engine.

The introduction of reagents into the flow of an exhaust gas of an IC engine prior to the gas passing through a catalyst in order to effect selective catalytic reduction (SCR) of NOx is well known.

The known systems principally fall into one of two categories, those which introduce gaseous ammonia into the exhaust conduit and those which introduce into the exhaust conduit a liquid reagent which decomposes into ammonia gas in the conduit. The introduction of gaseous ammonia into exhaust gasses for SCR purposes has been known for a long time in association with static systems, for example the after-treatment of flue gas in power plants. Over time, the benefit of SCR has been realised in mobile solutions, initially in the shipping industry and more recently in the motor vehicle industry. Where the application is mobile, for example a motor vehicle, there are, however, safety implications in carrying a large supply of ammonia on board because rupture of the ammonia storage vessel, for example in a crash, could cause the release of large volumes of ammonia into the atmosphere.

One solution to this problem has been to inject a liquid reagent into the hot exhaust gas where it decomposes into ammonia. The liquid reagent is, at ambient temperatures, a stable medium, but it decomposes at elevated temperatures to form at least ammonia gas. It is preferably an aqueous solution of urea or related substance such as biuret or ammonium carbamate, collectively referred to, and defined, herein as “urea”. While this solution to the problem provides a satisfactory result, there are a number of problems associated with it. Firstly, the liquid is injected through a nozzle as a fine spray of droplets into the fast flowing exhaust gas in which it preferably fully decomposes into at least ammonia gas prior to contacting the SCR catalyst. As this is not an instantaneous process, there needs to be a minimum separation distance between the injector and the SCR catalyst. Secondly is the problem of precipitation of solids from the urea solution, especially in the injector nozzle, which tends to occur particularly where dormant urea solution has resided at a high temperature under minimal pressure for a period of time in the injector nozzle. The solids may frequently block the nozzles, calling for complex control systems either to cool the nozzle or to re-circulate the urea so that it does not have the requisite time at elevated temperature for the precipitation to occur.

An alternative solution to the problem has been proposed in U.S. Pat. No. 6,361,754 and comprises hydrolysing aqueous urea under pressure at a high temperature so that it decomposes into at least gaseous ammonia and then introducing the gaseous ammonia into the exhaust conduit. While this is an efficient method of preparing ammonia gas in situ, as the heating is dependant on the reactor being placed in the exhaust conduit and the pressure under which the urea is being maintained will vary depending on the dosing of the gas into the exhaust, it is very hard to maintain a stable reaction. Also, all components of the system, of which there are many, need to be maintained at a minimum temperature and pressure to prevent the precipitation of solids. The pressure is linked to the dosing and if compensated by continual supply of aqueous urea then in times of peak demand the aqueous urea may pass fully through the reactor and be dosed directly into the exhaust. It is clear therefore that the system of U.S. Pat. No. 6,361,754 cannot be controlled to adequately match the rate of production of ammonia gas in the reactor to the demand for ammonia gas in the exhaust. U.S. Pat. No. 6,399,034 discloses an alternative solution which utilises decomposition of an alternative reagent, for example an aqueous solution of ammonium carbamate, which decomposes at a lower temperature, and stores the ammonia gas produced in an intermediate storage vessel and doses the gas from that vessel into the exhaust. The aqueous solution is heated by the engine cooling system which is capable of providing the lower heat requirements to decompose ammonium carbamate, but which would not be sufficient to hydrolyse urea at the rate required for NOx reduction in the exhaust of an IC engine. Furthermore, the reactor of U.S. Pat. No. 6,399,034 is not provided with a continuous supply of aqueous solution, rather the reactor contains a finite quantity of ammonium carbamate which depletes over time. It is not possible therefore to control the rate of reaction by metering the delivery of aqueous solution into the reactor.

It is a purpose of the present invention to mitigate some of the above problems by providing a simplified apparatus, and associated control methodology, for the production of ammonia gas for use in SCR systems of IC engines, especially but not exclusively diesel engines.

According to the present invention there is provided a device for generating gaseous hydrolysis product comprising ammonia, formed by the hydrolysis of an aqueous solution of urea (as hereinbefore defined) at elevated temperature and pressure, for feeding into the exhaust gas of an IC engine as it flows through the exhaust system of the engine, the device being adapted to be placed in the exhaust system so that the exhaust gas will flow through it during use, and comprising

a) a housing having an inlet for the exhaust gas and an outlet for the exhaust gas;

b) a reaction vessel located at least partially within the housing between the inlet and the outlet for containing an aqueous solution of urea and arranged such that, in use, the vessel and therefore the urea solution become heated by means of heat exchange with the exhaust gas as it flows from the inlet to the outlet;

c) a urea solution inlet to the reaction vessel and a gaseous hydrolysis product outlet from the reaction vessel;

d) a pump for pumping urea solution into the reaction vessel via the urea solution inlet; and

e) control means for controlling the pump in response to changing NOx output from the IC engine;

wherein, in response to an increase in said NOx output, the control means controls the pump to increase the level of urea solution in the reactor vessel, thereby increasing the surface area of urea solution available for heat exchange with the exhaust gas so as to increase the rate of production of gaseous hydrolysis product in the reactor vessel. Preferably, in response to a decrease in said NOx output, the control means controls the pump to decrease the level of urea solution in the reactor vessel, thereby decreasing the surface area of urea solution available for heat exchange with the exhaust gas so as to decrease the rate of production of gaseous hydrolysis product in the reactor vessel. Preferably, the device further comprises a sensor placed within the exhaust gas flow to measure the quantity of NOx therein.

Advantageously, the NOx sensor may be upstream or downstream of the SCR catalyst and would either measure the NOx output of the engine or the NOx output of the vehicle respectively. If the NOx output of the engine is measured then the signal is used to predict the required volume of the gaseous hydrolysis product required to be dosed into the gas to effect its removal (i.e. open loop control), whereas if the NOx output of the vehicle is sensed then more or less gaseous hydrolysis product will be dosed into the exhaust gas depending whether the sensed NOx level is above or below a target level (i.e. closed loop control)

Alternatively, engine management data, for example torque, engine speed, and/or throttle setting, are interrogated in order to deduce the NOx output of the vehicle.

Preferably, the device includes a reservoir for receiving and storing gaseous hydrolysis product. More preferably, the device includes a conduit for interconnecting the reservoir and the exhaust system. Most preferably, the conduit includes valve means to selectively control the feed of hydrolysis product stored in the reservoir into the exhaust gas via the conduit.

Preferably, level and/or temperature and/or pressure sensors are provided in the reactor.

Preferably, all the sensors required in the reactor are provided in a single cluster, removable in its entirety to minimise the number of access points required in the pressurised reactor. Preferably there is additionally a quality sensor provided in the reservoir and optionally in the urea storage tank to monitor the quality (for example the concentration) of the urea. Preferably the level sensor also acts as the quality sensor.

Preferably the device is provided with ammonia sensors downstream of the SCR catalyst to measure the ammonia slip. Preferably temperature sensors are provided inside the SCR catalyst to measure the temperature of the catalyst. Preferably there are also sensors provided upstream and/or downstream of the SCR catalyst to fully measure the temperature changes of the exhaust gas as it passes through the catalyst.

Preferably, the device includes a valve in the outlet from the reaction vessel, the valve being adapted to cause the contents of the reaction vessel, in use, to attain an elevated pressure as it becomes heated, and to discharge gaseous hydrolysis product into the reservoir

The valve may take a number of forms. In one preferred arrangement the valve actuates in response to the pressure within the reactor. This can be an active actuation where the pressure is measured in the reactor and the valve is actuated via a control system depending on the signal received from a pressure transducer situated in the reactor. Alternatively this can be a passive actuation where the valve is self actuating when a preset pressure occurs on its inlet side, i.e. it is a simple mechanical back pressure valve. In an alternative preferred arrangement the valve actuates in response

to the temperature of the aqueous solution of urea. This is preferably done by measuring the temperature within the aqueous urea solution and actuating the valve in response to the measured temperature. As the reaction occurs within the reaction vessel and the pressure rises the temperature within the solution also rises until both are elevated, and as there is a direct relationship between the two, control of the release of the gaseous hydrolysis product can be based on either. The valve for controlling the release of the gaseous hydrolysis product is preferably placed in the bulkhead between the reactor and the reservoir.

In a preferred arrangement the device further includes an auxiliary heating means for heating the reservoir, thereby enabling the reservoir to become heated prior to the engine being started, or alternatively enabling the reservoir to be maintained at an elevated temperature when the engine is switched off. The auxiliary heating means is preferably an electrically powered heater or a diesel burning heater. Preferably the device further comprises a bypass valve which can selectively control the proportion of the exhaust gas which is in thermal contact with the reactor to control the heat input into it.

Preferably the device is adapted for use with mobile, for example vehicle, engines. As the hydrolysis reaction favours fairly stable conditions then in such applications, and due to the transient operating conditions, it necessary to have a reservoir to store some of the hydrolysis product so the system can respond quickly to changes in the requirement for said hydrolysis product. This results in a residual volume of hot, pressurised, ammonia containing, hydrolysis gas in the reservoir when the engine is shut down. The content of the hydrolysis gas will depend on the reagent which is initially used which may for example be aqueous urea or ammonium carbamate. Both these reagents and a number of others will result in a hydrolysis gas containing steam and carbon dioxide as well as the ammonia. As the reservoir cools below 60 degrees, ammonia and carbon dioxide will react to form ammonium carbamate which will then at least partially dissolve in the water which forms as the steam condenses. Preferably the reservoir acts as a secondary reactor to, when the engine is re-started, heat the contents therein to evaporate the water and decompose the ammonium carbamate into the carbon dioxide and ammonia from which it formed.

Preferably there is a holding area into which, in response to a desire to reduce the liquid volume within the reactor, an amount of the aqueous urea is moved for temporary holding. Preferably the holding area is separate from the aqueous urea storage tank.

Preferably when it is required to increase the liquid-volume within the reactor, if there is any liquid in the holding area, the reactor is filled from the holding area until it is empty upon which, if further filling is required, the reactor will be filled from the aqueous urea storage tank.

Preferably the holding area is maintained at a temperature above which solids form within the liquid.

Preferably, both the reactor and the reservoir are heated by heat exchange with the exhaust gas.

Preferably, the device includes a catalyst arranged within the reactor to advance the rate of hydrolysis of the aqueous solution. More preferably the catalyst is arranged on a substrate. Most preferably the substrate is conical or frustoconical.

According to a second aspect of the invention there is provided a method of controlling the generation of a gaseous hydrolysis product comprising ammonia, and the feeding of that product into the exhaust gas of an IC engine, the method comprising the steps of:

a) providing a housing having an inlet for the exhaust gas and an outlet for the exhaust gas;

b) providing a reaction vessel located at least partially within the housing between the inlet and the outlet for containing an aqueous solution of urea and arranged such that, in use, the vessel and therefore the urea solution become heated by means of heat exchange with the exhaust gas as it flows from the inlet to the outlet;

c) providing a urea solution inlet to the reaction vessel and a gaseous hydrolysis product outlet from the reaction vessel;

d) providing a pump for pumping urea solution from into the reaction vessel via the urea solution inlet; the method further comprising the steps of:

e) hydrolysing an aqueous solution of urea (as hereinbefore defined) at elevated temperature and pressure within the reactor vessel;

f) determining the level of NOx in the exhaust gas;

g) controlling the pump to increase the level of urea solution in the reactor vessel in response to an increase in NOx levels in the exhaust system, thereby increasing the surface area of urea solution available for heat exchange with the exhaust gas so as to increase the rate of hydrolysis in the reactor vessel.

Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings in which;

FIG. 1 is a perspective view of the device in accordance with the invention;

FIG. 2 is a longitudinal cross section through the device of FIG. 1;

FIG. 3 is a transverse cross section through a first reaction vessel design of the device of FIG. 1;

FIG. 4 is a schematic representation of a control system including the device of FIG. 1;

FIG. 5 is a transverse cross section through a second reaction vessel design of the device of FIG. 1;

FIG. 6 is a longitudinal cross section through the device of FIG. 1 incorporated with an SCR catalyst;

FIG. 7 is a longitudinal cross section through the device of FIG. 1 incorporated within a housing, the housing containing the device, a particulate removal device and an SCR catalyst;

FIG. 8 is a perspective view of a second design of the device with variable heating;

FIG. 9 is a transverse cross section of the device of FIG. 8;

FIG. 10 is a perspective view of the device of FIG. 1 including a diesel particulate filter;

FIG. 11 is a perspective view of a third design of device according to the invention;

FIG. 12 is a perspective view of the rear of the device of FIG. 11;

FIG. 13 is a perspective view of the device shown in FIG. 11 with the outer cover removed;

FIG. 14 is a cross section through the reservoir and reaction vessel of FIG. 11;

FIG. 15 is a perspective view of a reservoir upper manifold according to the present invention;

FIG. 16 is an exploded assembly drawing of the reservoir upper manifold of FIG. 15 and its associated components;

FIG. 17 is a perspective view of the assembled reservoir upper manifold of FIG. 15 and its associated components;

FIG. 18 is a schematic representation of a control system according to the present invention.

Referring to FIGS. 1 to 4 a device 1 is shown capable of being placed in-line in the exhaust conduit of an IC engine, for example that found on a diesel vehicle, upstream of an SCR catalyst. The device 1 produces a gaseous product which is added to the exhaust gas in a controlled manner to pass therewith through the SCR catalyst to reduce the NOx content of the exhaust gas. The device 1 has an inlet 2 and an outlet 3 for the exhaust gas flowing therethrough and comprises an outer body 4, which forms a pressure barrier, and passing through the outer body 4 is an inner body 5 which comprises a flowpath longitudinally therethrough from the inlet 2 to the outlet 3. The device 1 is split into two sections, the first section is for hydrolysing an aqueous solution of urea at elevated temperature and pressure so that it decomposes to form a gaseous hydrolysis product containing ammonia gas. In the first section the outer 4 and inner 5 bodies form two chambers 6, 7 therebetween, substantially above the inner body and substantially below the inner body respectively. The lower chamber 7 has an inlet 8 for receiving a supply of aqueous urea solution delivered by a pump 32 (shown in FIG. 4 only) The upper chamber 6 has an outlet 9 for gaseous hydrolysis product.

The lower 7 and upper 6 chambers are connected by a plurality of tubular elements 10 which pass through the inner body 5 and which form fluid flowpaths between the lower 7 and upper 6 chambers. The upper 6 and lower 7 chambers and the tubular elements 10 together form and enclosed reaction vessel in which the hydrolysis reaction occurs.

In use the aqueous solution of urea is fed into the reaction vessel via the inlet 8 in the lower chamber 7 by the pump 32. The level of aqueous urea in the reaction vessel is measured by a level sensor 11. Although the level sensor 11 is shown to be only in the upper chamber 6, for greater control over the liquid level within the reaction vessel it may extend into the lower chamber 7 through one of the passageways or alternatively a second level sensor 12 may be placed in the lower chamber (FIGS. 3 and 4).

The exhaust gas from the engine, which has a temperature up to around 550 degrees centigrade (dependent on engine load), passes over the tubes 10 and the upper and lower surfaces of the lower 7 and upper 6 chambers respectively, raising the temperature of the liquid contained therein by heat exchange. As the temperature rises the hydrolysis reaction starts to occur (at approximately 60 degrees centigrade) and the gaseous hydrolysis product starts to collect in the headspace above the liquid level in the upper chamber 6. As the temperature rises further the reaction accelerates and a head of pressure builds up in the head space, pressurising the reaction vessel and allowing the temperature of the aqueous urea solution to rise above the temperature at which it would otherwise boil. The reaction vessel outlet 9 in the upper chamber 6 includes a valve 13 which opens passively at a predetermined set pressure, preferably in the region of 15 to 20 bar, ideally 17 bar. Thus the pressure in the reaction vessel is elevated above atmospheric pressure but is maintained below a certain value (in this case 17 bar), which gives a good reaction rate without the need to contain excessive pressures.

Alternatively the valve 13 may be active, i.e. it may operate in response to a pressure sensor 14 within the header section of the reservoir 15 (as will be described in further detail shortly).

The valve 13 releases the excess pressure from the reaction vessel into the second section of the device which comprises a reservoir 15 which surrounds the inner body 5. The passage of hot exhaust gas through the inner body 5 heats the reservoir and keeps the ammonia containing hydrolysis product in its gaseous state. The reaction vessel has an outlet 16 and a dosing valve 17 associated therewith. The device is further provided with a pressure sensor 18 to sense the pressure in the reservoir 15. Referring now to FIG. 4, the pump 32 delivers aqueous urea solution from a holding tank 35 into the lower chamber 7 via the inlet 8. The pump 32 is controlled by a controller 33 which is also connected electrically to the level sensors 11, 12; reaction vessel outlet valve 13; dosing valve 17; reaction vessel pressure sensors 14, 18; and an engine management system 34 as indicated by the dashed lines in FIG. 4. The engine management system 34 logs and controls the performance characteristics of the IC engine in known manner.

The device 1 is operable as follows. The controller 33 receives a supply of data from the engine management system 34, the data including, for example, engine speed, torque, ignition timing and throttle position. This data is used to calculate the NOx level in the engine exhaust according to known techniques, such as executing algorithms on the engine management data or referencing look up tables. Given the NOx level in the exhaust, the controller 33 then calculates the volume of ammonia gas required to react with the prevailing level of NOx established in the exhaust. Accordingly, in times of increased engine demand, for example high engine speed and/or torque, the controller 33 controls the pump 32 to increase the rate of delivery of aqueous solution into the reaction vessel. This results in an increase in the level of aqueous solution within the reaction vessel. Thus, a greater surface area of the inside of the reaction vessel becomes wetted by the aqueous solution. The resulting increase in the heated wetted area in the reactor vessel, ie the total surface area of aqueous solution directly exposed to heat from the exhaust, causes increased heat transfer from the exhaust gas to the aqueous solution. This in turn generates an increased rate of production of gaseous hydrolysis product.

In this manner, the controller 33 delivers an increased volume of aqueous solution into the reaction vessel in response to an increase in the level of NOx in the exhaust gas. Conversely, in times of decreased engine demand, the controller 33 controls the pump 32 to decrease the rate of delivery of aqueous solution into the reaction vessel. This results in a reduced rate of production of gaseous hydrolysis product. In times of increased load, the NOx levels in the exhaust increase. This increases the demand for ammonia gas, in response to which the controller 33 controls the pump 32 to increase the rate of delivery of aqueous solution to the reaction vessel. However, increased engine load also delivers an increase in exhaust gas temperature and flow rate. In particular, a high engine speed will lead to a high exhaust gas flow rate and high torque operation will increase the exhaust gas temperature. Consequently, in times of high load, an increase in the heated, wetted area of aqueous solution in the reaction vessel is observed concurrently with an increased exhaust gas temperature and/or flow rate. Accordingly not only is the rate of production of gaseous hydrolysis product increased by virtue of the increased heated wetted area, but also by the increased rate of heat transfer per unit area delivered by the increase in exhaust gas temperature and/or flow rate. In this way the increased volume of aqueous solution in the reactor vessel is balanced by the increased rate of gaseous hydrolysis product. This leads to a reduction in the volume of aqueous solution and thereby a stabilisation in the level of aqueous product within the reaction vessel.

However, there exist engine operating conditions where the increased rate of delivery of aqueous solution is not matched by an increased exhaust gas temperature and/or pressure. For example, where an engine is under high load driving the vehicle up a steep incline, NOx levels in the exhaust will increase. However, upon reaching the end of the incline the engine may operate at tick over or very low load, for example in negotiating a decline. The exhaust temperature will consequently decrease leading to a mismatch in exhaust conditions and rate of aqueous solution delivery into the reaction vessel. Under such circumstances, the controller 33 controls the pump 32 to pump aqueous solution from the reaction vessel into a holding reservoir (not shown for clarity). The solution is held in the holding vessel until such time as the demand for ammonia increases at which point the controller 33 controls the pump 32 to pump the solution from the holding vessel into the reaction vessel. The holding vessel may be heated by an auxiliary heating means in order to prevent condensation of the gaseous hydrolysis product. The holding vessel is evacuated before the pump pumps aqueous solution from the tank in order to retrieve the heat retained in the solution in the holding vessel by virtue of its earlier heating in the reaction vessel. In the event that engine load rapidly increases following a period of low load, for example beginning a steep incline having previously completed a low load descent, the second level sensor 11 is provided to ensure that the level of aqueous solution in the reaction vessel does not become dangerously low.

As discussed above the gaseous hydrolysis product is released into the reservoir 15 when the headspace pressure in the reaction vessel rises above 17 bar. The dosing of the gas from the reservoir is controlled as follows. The valve 17 is operable in response to a signal from the controller 33 to open and allow some of the gas within the reservoir 15 to enter the exhaust gas flowing through the inner body 5 to flow therewith through an SCR catalyst (not shown) positioned downstream of the device. The controller 33 monitors the reservoir pressure via pressure sensor 18 and calculates the required opening of the valve (for the given pressure) to introduce the required volume of hydrolysis product (or component thereof) dictated by the engine exhaust conditions. Optionally the reservoir temperature is also monitored as will be discussed in further detail shortly.

Accordingly, the reservoir 15 acts as a buffer between the reaction vessel and the IC engine exhaust. The reservoir depletes and replenishes so as to allow for the lag in the control of the rate of production of gaseous hydrolysis product in response to the prevailing exhaust conditions.

Referring to FIG. 5 an alternative arrangement of the reaction vessel section is shown in which the inner body 5 sits within the outer body 4 and is arranged such that there is an upper chamber 6 and a lower chamber 7 substantially above and below the inner body 5, and a passageway 19 formed between the walls of the inner 5 and outer 4 chambers making them in fluid communication. The reaction vessel acts in substantially the same manner as described in reference to FIGS. 1 to 4 and the fluid is heated as it passes from the lower to the upper chamber by conduction through the walls of the inner body 5. Optionally a combination of the two designs may be utilised whereby the fluid passes from the lower chamber 7 to the upper chamber 8 through the tubes 10 (see FIG. 3) and through the passageways 19.

Referring to FIG. 6 the device as described in relation to FIGS. 1 to 4 is shown incorporated with an SCR Catalyst 20. The SCR catalyst 20 is contained within an open ended housing 21 which fits around the outer body 4 of the device. It may be attached by any means known in the art, for example a simple screw thread or bayonet type fitting. The outlet 3 of the device feeds directly into the SCR catalyst, the exhaust gasses passing therethrough and exiting the extended form of the device via SCR outlet 22. In an alternative arrangement (not shown) the outer wall 4 of the device extends beyond the reservoir 15 and in its extended region forms a housing for the SCR Catalyst.

Referring to FIG. 7 the device as described in relation to FIGS. 1 to 4 is shown incorporated within a common housing with the SCR catalyst and a particulate removal device. The common housing 23 has an inlet 24 and an outlet 25 and located therebetween a particulate removal device 26, the device of the invention 27 and the SCR catalyst 28. The particulate removal device may be any such device known in the art, for example a diesel particulate filter.

Referring to FIGS. 8 and 9 the device as described in FIGS. 1 to 4 is shown which additionally comprises a heat exchange bypass. The inner body 5 is split in two longitudinally by means of a dividing plate 29 to create a heat exchange section containing the tubes 10 and a bypass section 30, and a diverter flap 31 is placed upstream of dividing plate 29. The diverter flap 30 is movable by controller 100 (see FIG. 4) to selectively allow a varying amount of the exhaust gas to flow over the tubes 10 thereby controlling the heat input into the reaction vessel. This enables the speed at which the hydrolysis occurs, and therefore the speed at which the ammonia-containing gas is produced, to be controlled by way of the position of the diverter plate in addition to, or instead of, the rate of delivery of aqueous solution to the reaction vessel.

Referring to FIG. 10 a gas treatment apparatus is shown particularly useful for treating the exhaust gas of a commercial vehicle engine. An outer housing comprising endplates 41, 42 is split into three sections by plates 43 and 44 forming two end sections 45, 46 and a central section 47. An exhaust inlet 48 passes through plate 41 and section 45 and opens into an oxidation catalyst 49 situated in central section 47 and extending between plates 44 and 45. The outlet of the oxidation catalyst passes through plate 44 opening into end section 46, the exhaust gas expanding as it does so.

Also located in the central section 47 between plates 43 and 44 are two SCR catalysts 50, 51 their inlets being in end section 46 and their outlets discharging into end section 45 such that the exhaust gas entering end section 46 then passes through the SCR catalysts into end section 45. Situated in plate 44 and leading from end section 45 is an inlet into a closed end baffle drum 52 which has a number of outlets 53 in the side of the drum opening into central section 47. Also located in central section 47 is a second closed end baffle drum 54 which has a plurality of inlets 55 in the side of the drum 54 and an outlet 56 leading from the drum 54 and passing through end section 46 and out of the apparatus for discharge to atmosphere. Located within end section 46 is a hydrolysis reaction vessel 57 as described in relation to FIG. 10 which has an inlet 58 for a pressurised supply of urea and an outlet 59 for ammonia containing gas. The reaction vessel 57 is heated by heat exchange with the exhaust gas circulating within the end section 46 as it passes from the outlets of the oxidation catalyst 49 to the inlets of the SCR catalysts 50, 51. Also located within end section 46, but separated from the gas flow therein by a baffle plate 60, is a valve unit 61 containing a pressure control valve for controlling the pressure within the reaction vessel and a dosing valve for controlling the flow of ammonia containing gas to two injection points 62. Each injection point 62 injects ammonia containing gas into the exhaust gas stream prior to it passing through the two SCR catalysts 50, 51 wherein the ammonia reacts with the NOx in the exhaust as on the surface of the SCR catalysts 50, 51 reducing the NOx content of the exhaust to a level acceptable for discharge to atmosphere via outlet 56.

The pressure control valve of the valve unit 61 has an outlet leading a gas reservoir 63 which provides a buffer of ammonia containing gas ready to be dosed into the exhaust gas via the dosing control valve of the valve unit 61 and the injection points 62. The gas reservoir 63 is situated in the central section 47 of the apparatus in which the exhaust gas passing from baffle drum 52 to baffle drum 54 is circulating and is thereby heated by heat exchange with the hot exhaust gas.

Referring to FIGS. 11 to 14, an alternative embodiment of gas treatment device 64 is shown which operates in a substantially similar manner to the embodiment described previously. The exhaust gas of an IC engine flows through the device 64 from an inlet 65 to an outlet 66. The exhaust enters the inlet 65 containing NOx and leaves the outlet 66 substantially free on NOx. The device 64 may be attached to a commercial or passenger vehicle and connected in line in the existing vehicle exhaust system. When the exhaust gas passes through the inlet 65 it passes a NOx sensor 112 before entering a first cylindrical tube 67 containing a hydrolysis reaction vessel 68 (see FIG. 14). The hot exhaust gasses exit the tube 67 through an opening therein and enter an enclosed cavity 69. As the hot exhaust gasses pass over the reaction vessel 68 it absorbs heat from the gasses and becomes elevated in temperature. The reaction vessel 68 has an inlet 70 at its lower end through which an aqueous solution of urea is supplied. The aqueous solution is delivered from a holding tank 110 by a pump 111, both shown schematically in FIG. 14 only.

As the reaction vessel 68 becomes heated the aqueous solution of urea starts to hydrolyse and hydrolysis gasses form in the head space above the level of the urea. The reaction vessel 68 is provided with a pressure relief valve 71 in its upper end which allows the hydrolysis gas to pass from the reaction vessel 68 to a reservoir 72 if the pressure in the reaction vessel 68 exceeds 17 bar. The tube 67 has a closed upper end (with an opening therein through which the pressure relief valve 71 projects). The reaction vessel 68 is attached to the device by its upper end.

The enclosed cavity 69 has a passageway in one of its walls (not shown) allowing the exhaust gas to exit the cavity 69 and pass through an oxidation catalyst 74 where a percentage of the NO in the exhaust gas is oxidised into NO₂. The exhaust gas then exits the oxidation catalyst and enters a truncated conical section 75 which reduces in diameter.

A feed tube 76 leads from the reservoir into the conical section 75 and the hydrolysis gas is dosed through the feed tube 76 into the exhaust gas at the open end of the cone. As the flow area reduces mixing is induced between the exhaust gas and the hydrolysis gas. After the conical section 75 the exhaust gasses pass around a 90° bend 82 and flows into a cylindrical vortex mixer 83. The exhaust gasses enter the vortex mixer 83 tangentially and exit along its central axis into an SCR catalyst 84 wherein the hydrolysis gas mixes with the NOx converting it substantially to nitrogen and water. The exhaust gas exits the SCR catalyst 84 and expands into the interior of the device enclosed by cover 85. The treated exhaust gasses then exit the device via the outlet 66 which passes through the enclosed cavity 69. Arranged in proximity to the exit 66 are a NOx sensor 113 and an ammonia sensor 114.

The flow of hydrolysis gas from the reservoir 72 into the conical section 75 via the tube 76 is controlled by a dosing valve 77 (as will be described in further detail shortly) attached to an upper manifold 78 of the reservoir 72. The reservoir 72 is located in a tube 79 and positioned such that there is an air gap between the reservoir 72 and the tube 79. Part of the outer surface of the tube 79 forms a wall of the enclosed cavity 69 and as such is in direct contact with the hot exhaust gasses. In use the reservoir becomes heated by heat transfer from the exhaust gas through the tube 79 and across the air gap. The reservoir 72 is elongate in shape and similar to the reaction vessel 68 will expand in length. The reservoir 72 is attached at its upper end and free to expand at its lower end. A sliding seal 80 is provided to retain the lower end of the reservoir 72. A heater 81 is situated at the lower end of the reservoir to allow for additional heating to supplement the heat from the exhaust gasses. The pressure release valve 71 and the dosing valve 77 are maintained in a cooler area and are separated from the warmer area by a manifold plate 86, which may either be of a thermally shielding material or may include a thermal shield. The pressure relief valve 71 and the dosing valve 77 have covers 87, 88 sealed thereover maintaining them in a clean and dry environment.

Referring to FIGS. 15 to 17, a reservoir upper manifold 89 for use in a gas reservoir containing ammonia and carbon dioxide for use in an SCR process is shown. It can be used, for example, as the reservoir in the system described with reference to FIGS. 12 to 14. The upper manifold forms the upper end of the reservoir 72 (see FIG. 14) and is welded to the reservoir body 90. The upper manifold 89 has a plurality of passageways in it adapted to accommodate associated components. Passageway 91 is the gas inlet to the reservoir and is fed with a supplied of ammonia containing gas via tube 92. Passageway 93 is the inlet for a dosing valve 94. The gas from the reservoir enters the valve 94 through port 93 and exits through pipe 95 which passes back into the reservoir and passes through the reservoir body 90 via a bulkhead fitting (not shown). The dosing valve 94 is operable to control the flow of ammonia containing gas from the reservoir into the exhaust gas flow of an IC engine. Passageway 96 accommodates a safety valve 97 which opens above a preset pressure and vents excess gas pressure out of the passageway 96, past the safety valve 97 and sideways out through passageway 98 from where it flows through a tube 99 into the exhaust gas flow.

Passageway 100 accommodates a reservoir pressure sensor 101. Passageway 102 accommodates a fitting 104 to accept a reservoir temperature sensor 103 which detects the temperature of the gas within the reservoir. The same sensor (or a second sensor) can also monitor the temperature of the upper manifold itself.

The upper manifold 89 has a plurality of ports 105 in its sides to accommodate heating elements 106. If the ammonia and carbon dioxide gasses cool down in the presence of each other then they can form solid salts, e.g. ammonium carbamate, which can block the valves resulting in not only the inability to dose the gas into the exhaust gas but also a possibly dangerous increase in pressure within the reservoir. Alternatively a build up of solids may occur on the sensors 101, 103 causing them to malfunction, again possibly leading to a dangerous increase in pressure within the reservoir. The heaters 106 are operated to maintain the upper manifold 89 at a raised temperature to prevent solidification of any salts in any of the passageways therethrough. The heaters maintain the upper manifold 89 above 130° C., ideally at a substantially constant temperature of 220° C. Between the upper manifold and the components is a thermal barrier 107 to protect the components from heat radiated directly from the upper manifold. The upper manifold has a number of threaded holes 108 therein for attaching a cover 109 to it. The thermal barrier also acts as a gasket and seals the cover 109 over to the manifold, thus the reservoir can be washed, for example with a powerful spray of water, without water ingress into the associated components 92, 94, 101, 104 and any associated electronics. The cover 109 is made of aluminium has a plurality of cooling fins to assist in rapid heat loss from this section maintaining the components within their working temperature range.

In use the device 64 is operable as follows. A controller (not shown in FIGS. 11 to 17 for clarity) is provided which is electrically connected to the dosing valve 77, reservoir pressure sensor 101, reservoir temperature sensor 103, heaters 81, 106, NOx sensors 112, 113 and ammonia sensor 114.

The device 64 may optionally be provided with an analogue level sensor for measuring the exact level of aqueous solution in the reaction vessel 68, the level sensor also being connected electrically to the controller. The reaction vessel may also have optional temperature and pressure sensors for communicating to the controller the reaction vessel conditions in order to control an active pressure release valve in place of the passive unit described above.

In distinction to the first embodiment, the controller of the second embodiment receives a signal from the NOx sensor 112 rather than calculating the exhaust NOx levels by derivation from engine load data.

The volume of ammonia gas required to react with the NOx level detected in the exhaust gas is calculated and the pump 111 controlled accordingly to increase or decrease the rate of flow of aqueous solution into the reactor. The level of aqueous solution in the reaction vessel rises or lowers accordingly, thereby altering the rate of heat transfer between the aqueous solution and the exhaust gas as described previously.

The controller also monitors downstream NOx levels in the exhaust by way of NOx sensor 113 in order to ensure that NOx consumption is maximised. Like wise the controller monitors ammonia levels in the exhaust gas exiting the device 64 by way of an ammonia sensor 114 in order to minimise the risk of ammonia slip. The controller also monitors the reservoir temperature and pressure by way of temperature sensor 103 and pressure sensor 101. When the reservoir temperature and/or pressure fall below predetermined values, the reservoir heater 81 is operated to raise the reservoir temperature in order to prevent the gaseous hydrolysis product condensing.

This is particularly advantageous at cold start-up of the IC engine as the residual condensate in the reservoir is heated to provide ammonia for delivery into the exhaust before the exhaust gas has raised the temperature of the reactor sufficiently to cause hydrolysis of the aqueous solution.

In a further embodiment the reaction vessel includes a conical catalyst substrate. The varying cross-sectional area of the substrate with height further emphasises the effect of altering the rate of hydrolysis by changing the level of aqueous solution in the reaction vessel. Alternatively, the substrate may have a form other than conical, for example cylindrical in order to deliver a particular change in reaction rate per unit increase in the liquid height.

Referring now to FIG. 18, a control methodology 200 is shown for controlling the devices 1, 64 described above. The methodology is described hereafter with reference to the gas treatment device 64 but is equally applicable to the gas treatment device 1. A demand generator 202 receives a catalyst condition signal 204 from the exhaust catalyst, a NOx sensor signal 206 from the NOx sensor 112, an engine condition signal 208, from the engine management system (not shown for clarity) and a demand signal 210.

The demand generator calculates a required ammonia output rate and delivers an ammonia output signal 212. The ammonia output signal is delivered to a dosing valve control 214 and a pump control 216. The dosing valve control 214 outputs a dosing valve signal 218 to command the opening and closing of the dosing valve 77. In order to calculate the required dosing valve signal 218, the dosing valve control 214 receives a reservoir pressure signal 220 and a reservoir temperature signal 222 from the reservoir pressure sensor 101 and reservoir temperature sensor 103. The reservoir pressure signal 220 is also delivered to the pump control 216 in addition to an integral and differential of the pressure signal. The pump control 216 outputs a pump signal 224 to control the pump 111. In order to generate the pump signal 224, the pump control 216 may optionally also receive a reactor level signal 226 from a reactor level sensor (not shown for clarity). The reservoir temperature signal 222 is also delivered to a reservoir heater control 228 which generates a reservoir heater signal to control the reservoir heater 81.

Optionally, a reactor pressure sensor delivers a reactor pressure signal 232 to a reactor pressure control 234 which outputs a pressure relief valve signal 236 to an active pressure relief valve for venting gaseous hydrolysis product from the reactor into the reservoir. This optional control methodology is only required when an active pressure relief valve is used in place of a passive valve.

It will be appreciated that various components and control methods are described in respect of one or other of the embodiments. Nonetheless any of the measurement and control features described above are interchangeable between embodiments. It will also be appreciated that within the scope of the invention various components described herein with reference to one or other of the embodiments are interchangeable. For example systems falling within the scope of the invention may include a combination of features not explicitly described in respect to any particular embodiment. 

1. A device for generating gaseous hydrolysis product comprising ammonia, formed by the hydrolysis of an aqueous solution of urea (as hereinbefore defined) at elevated temperature and pressure, for feeding into the exhaust gas of an IC engine as it flows through the exhaust system of the engine, the device being adapted to be placed in the exhaust system so that the exhaust gas will flow through it during use, and comprising a) a housing having an inlet for the exhaust gas and an outlet for the exhaust gas; b) a reaction vessel located within the housing between the inlet and the outlet for containing an aqueous solution of urea and arranged such that, in use, the vessel and therefore the urea solution become heated by means of heat exchange with the exhaust gas as it flows from the inlet to the outlet; c) a urea solution inlet to the reaction vessel and a gaseous hydrolysis product outlet from the reaction vessel; d) a pump for pumping urea solution into the reaction vessel via the urea solution inlet; e) control means for controlling the pump in response to changing NOx output from the IC engine; wherein, in response to an increase in said NOx output, the control means controls the pump to increase the level of urea solution in the reaction vessel, thereby increasing the surface area of urea solution available for heat exchange with the exhaust gas so as to increase the rate of production of gaseous hydrolysis product in the reactor vessel.
 2. The device of claim 1 including an SCR catalyst downstream of the outlet.
 3. The device of claim 1 wherein the device includes a NOx sensor arranged in the exhaust gas flow in proximity to the inlet for measuring the quantity of NOx in the untreated gas flow.
 4. The device of claim 2 wherein the device includes a NOx sensor arranged downstream of the SCR catalyst, the sensor for measuring the NOx output from the exhaust system.
 5. The device of claim 1, wherein the control means interrogates engine management data in order to deduce the NOx output from the engine.
 6. The device of claim 1, wherein the device includes a reservoir for receiving gaseous hydrolysis product from the reaction vessel and storing said product.
 7. The device of claim 6 wherein the device includes a conduit for interconnecting the reservoir and the exhaust system.
 8. The device of claim 7 wherein the conduit includes valve means to selectively control the feed of hydrolysis product stored in the reservoir into the exhaust gas.
 9. The device of claim 8 wherein the control means controller controls the valve means in order to dose a volume of gaseous hydrolysis product to match the NOx output from the IC engine.
 10. The device of claim 1 wherein the reaction vessel includes a level sensor for monitoring the level of aqueous solution in the reaction vessel.
 11. The device of claim 1 wherein the reaction vessel includes a quality sensor for monitoring the quality of the aqueous solution in the reaction vessel.
 12. The device of claim 10 wherein the level sensor also acts as a quality sensor for monitoring the quality of the aqueous solution in the reaction vessel.
 13. The device of claim 1 wherein the reaction vessel includes a pressure sensor for monitoring the pressure of the aqueous solution in the reaction vessel.
 14. The device of claim 1 wherein the reaction vessel includes a temperature sensor for monitoring the temperature of the aqueous solution in the reaction vessel.
 15. The device of claim 1, wherein the reaction vessel includes a reaction vessel sensor, said reaction vessel sensor is provided in a single cluster.
 16. The device of claim 1, including an SCR catalyst downstream of the outlet wherein the device is provided with ammonia sensors downstream of the SCR catalyst for measuring quantity of ammonia in the exhaust system outlet.
 17. The device of claim 1, including an SCR catalyst downstream of the outlet wherein a temperature sensor is provided inside the SCR catalyst to measure the temperature of the catalyst.
 18. The device of claim 1, including an SCR catalyst downstream of the outlet wherein a temperature sensor is provided upstream and/or downstream of the SCR catalyst.
 19. The device of claim 1, wherein the device includes a valve at the outlet from the reaction vessel, the valve being adapted to cause the contents of the reaction vessel, in use, to attain an elevated pressure as it becomes heated, and periodically to discharge gaseous hydrolysis product into the reservoir.
 20. The device of claim 19, further comprising a pressure sensor for monitoring the pressure of the aqueous solution in the reaction vessel wherein the valve is controlled by the control means to actuate in response to the control means detecting a predetermined pressure within the reaction vessel.
 21. The device of claim 19 wherein the valve is self actuating, discharging gaseous hydrolysis product into the reservoir when a preset pressure is achieved within the reaction vessel.
 22. The device of claim 19 wherein the reaction vessel includes a temperature sensor for monitoring the temperature of the aqueous solution in the reaction vessel and wherein the control means controller actuates the valve in response to the temperature of the aqueous solution in the reactor.
 23. The device of claim 1, wherein the device includes a reservoir for receiving gaseous hydrolysis product from the reaction vessel and storing said product and wherein the device includes an auxiliary heating means for heating the reservoir.
 24. The device of claim 23 wherein the auxiliary heating means is an electrically powered heater, or a diesel burning heater.
 25. The device of claim 1 wherein the device includes a bypass valve which can selectively control the proportion of the exhaust gas which is in thermal contact with the reaction vessel in order to control the heat input into the reaction vessel.
 26. The device of claim 1, wherein the device includes a reservoir for receiving gaseous hydrolysis product from the reaction vessel and storing said product and wherein the reservoir acts as a secondary reaction vessel to, when the IC engine is re-started, heat the contents therein to hydrolyse an aqueous solution condensate.
 27. The device of claim 1 wherein in response to a decrease in said NOx output, the control means controls the pump to decrease the level of urea solution in the reaction vessel, thereby decreasing the surface area of urea solution available for heat exchange with the exhaust gas so as to decrease the rate of production of gaseous hydrolysis product in the reactor vessel.
 28. The device of claim 1 wherein the device includes a holding vessel into which, in response to a decrease in said NOx output, the control means controls the pump to remove a volume of the aqueous urea from the reaction vessel for temporary holding in the holding vessel.
 29. The device of claim 28 wherein, when, in response to a subsequent increase in said NOx output, the controller controls the pump to pump any liquid in the holding vessel into the reaction vessel.
 30. The device of claim 28 wherein the holding vessel is maintained at a temperature above which solids form within the aqueous solution.
 31. The device of claim 1, wherein the device includes a reservoir for receiving gaseous hydrolysis product from the reaction vessel and storing said product and wherein both the reaction vessel and the reservoir are heated by heat exchange with the exhaust gas.
 32. The device of claim 1 wherein the device includes a catalyst arranged within the reaction vessel to advance the rate of hydrolysis of the aqueous solution.
 33. The device of claim 32 wherein the catalyst is arranged on a substrate.
 34. The device of claim 33 wherein the substrate is conical or frustoconical.
 35. A method of controlling the generation of a gaseous hydrolysis product comprising ammonia, and the feeding of that product into the exhaust gas of an IC engine, the method comprising the steps of: a) providing a housing having an inlet for the exhaust gas and an outlet for the exhaust gas; b) providing a reaction vessel located within the housing between the inlet and the outlet for containing an aqueous solution of urea and arranged such that, in use, the vessel and therefore the urea solution become heated by means of heat exchange with the exhaust gas as it flows from the inlet to the outlet; c) providing a urea solution inlet to the reaction vessel and a gaseous hydrolysis product outlet from the reaction vessel; d) providing a pump for pumping urea solution from in to the reaction vessel via the urea solution inlet; the method further comprising the steps of: e) hydrolysing an aqueous solution of urea (as hereinbefore defined) at elevated temperature and pressure within the reactor vessel; f) determining the level of NOx in the exhaust gas; g) controlling the pump to increase the level of urea solution in the reactor vessel in response to an increase in NOx levels in the exhaust system, thereby increasing the surface area of urea solution available for heat exchange with the exhaust gas so as to increase the rate of hydrolysis in the reactor vessel.
 36. The method of claim 35 including the steps of: providing reservoir for receiving gaseous hydrolysis product from the reaction vessel and storing said product; providing an auxiliary heating means for heating the reservoir;
 37. The method of claim 36 further including the step of: controlling the auxiliary heating means to heat the reservoir at cold start-up of the IC engine in order to deliver gaseous hydrolysis product from the reservoir into the exhaust gas before the reactor temperature and pressure is elevated sufficiently by the exhaust gas to hydrolyse the aqueous solution of urea contained therein.
 38. The method of claim 36 further including the step of: controlling the auxiliary heating means to ensure that the temperature and pressure in the reservoir is maintained at a temperature above which solids form within the aqueous solution during normal operation of the IC engine.
 39. The method of claim 35 further including the step of: providing a holding vessel for temporarily receiving aqueous solution from the reaction vessel; controlling the pump to pump aqueous solution from the reaction vessel into the holding vessel in response to a decrease in NOx output.
 40. The method of claims 39 further including the step of: providing an aqueous solution tank for containing a volume of aqueous solution; controlling the pump to pump aqueous solution from the holding vessel into the reaction vessel in response to an increase in NOx output so as to empty the holding vessel before controlling the pump to further increase the level of urea solution in the reactor vessel from the aqueous solution tank as necessary. 